1. Field of Invention
The present invention relates to a device for balanced-to-unbalanced line transformation (balun) and more particularly to a space-optimized balun that can be printed on a circuit board.
2. Description of Related Art
A balun is a device used to convert between balanced and unbalanced lines for input and output in an electrical system. Special considerations apply to the application of a balun to microwave systems that include printed circuit boards. As is commonly known in the art, FIG. 7 illustrates a ring or xe2x80x9cratracexe2x80x9d design that is used in printed circuit boards. The ring balun 72 is made from microstrip line 74, including a conductive material such as copper. (Microwave Circuit Design, G. D. Vendelin, A. M. Pavio, and U. L. Rohde, John Wiley and Sons, 1990).
For the unbalanced line the ring balun 72 includes a single-ended port 76 and an isolation port 78. For the balanced line the ring balun 72 includes a first differential port 80 and a second differential port 82.
The distances along the microstrip 72 between the ports is related to the operational wavelength xcex. As shown in FIG. 7 in a clockwise direction, the distance (measured circumferentially) between the single-ended port 76 and the first differential port 80 is xcex/4, the distance between the first differential port 80 and the isolation port 78 is xcex/4, the distance between the isolation port 78 and the second differential port 82 is xcex/4, and the distance between the second differential port 82 and the single-ended port 76 is 3xcex/4. In typical operation, the single-ended port 76 is driven by a signal at an operational frequency ƒ and a 50 xcexa9 resistor is attached to the isolation port 78. Then a differential signal is obtained from difference of the outputs at the first differential port 80 and the second differential port 82.
For the ring balun 72 the operational wavelength xcex is related to the operational frequency ƒ through the relation                     λ        =                  c                      f            ⁢                                          ϵ                r                                                                        (        1        )            
where c is the speed of light and xcex5r is a substrate dielectric constant associated with the microstrip 74. Typically the operational frequency ƒ is fixed by the application and there is only limited choice for the properties of the microstrip 74.
For example, for the case where ƒ=5.3 GHz and xcex5r=3.38 (e.g., for Rogers material RO4003(copyright), then the circumferential distance between the single-ended port and the open ended port is approximately xcex/4=350 mils. The ring balun 72 then approximately has a diameter of 668 mils and covers an area of 0.35 inch2. This balun 72 can be approximately contained within a square having a side of length 668 mils and having an area of 0.45 inch2.
The desirability of reducing the space occupied by elements on circuit boards has led to limited attempts to reduce the space occupied by the ring balun 72 by some modification of the geometry while keeping the essential features of the design. A difficulty with modifying the geometry of the ring balun 72 may arise due to interference (or coupling) between segments of microstrip that are relatively close together. This interference may adversely affect performance of the balun.
For example, FIG. 8 shows a modified ring balun 84 also made from microstrip line 86 and also having a single-ended port 88, an isolation port 90, a first differential port 92 and a second differential port 94. The circumferentially measured distances between the ports (88, 90, 92, 94) for the modified ring balun 84 are prescribed in terms of the wavelength xcex as in the ring balun 72. However, the arc between the first differential port 92 and the second differential port 94 is inverted, thereby saving some space on the circuit board while causing minimal interference near the cusps formed at the first differential port 92 and the second differential port 94. However, this improvement is minimal since the approximate area of a square that contains the modified balun 84 is still 0.447 inch2.
Thus, the requirements for the space taken by a printed balun on a circuit board are driven in part by the desired operational frequency and the physical properties of the microstrip. Attempts to modify the conventional ring balun design have led to limited improvements in minimizing the required area on a circuit board.
Accordingly, it is an object of this invention to provide a balun that can be printed on a circuit board to optimize the covered space.
It is a further object of this invention to provide a printed balun that is designed to perform at a prescribed operating frequency including microwave frequencies.
It is a further object of this invention to provide a printed balun that satisfies performance criteria for signal attenuation and return loss.
The above and related objects of the present invention are realized by a balun that satisfies performance requirements while minimizing the corresponding area required on a circuit board.
According to one aspect of the invention, the balun includes a single-ended port, an isolation port, a first differential port, a second differential port, and a microstrip. The microstrip defines a plurality of fingers including a first finger that connects to the single ended port, a second finger that connects to the isolation port, a third finger that connects to the first differential port, and a fourth finger that connects to the second differential port.
The microstrip may also define a central segment that is transverse to the fingers and thereby connects them. Preferably the angles formed by the microstrip are approximately ninety degrees so as to minimize the overall space required by the balun by allowing uniform separations between segments of the microstrip. The lengths of the segments can be tuned to operate adequately at desired frequencies such as 5.3 GHz and 4.2 GHz.